1. Field of the Invention
The present invention relates to a current source circuit using a bandgap voltage circuit.
2. Description of the Related Art
Current sources using bandgap voltage circuit are known in the art for example from U.S. Pat. No. 5,581,174.
The present invention provides embodiments which have improved power supply rejection and which allow for turning on and off the current generator in a simple manner. Further embodiments allow for multiple current outputs.
According to an embodiment of the present invention there is provided a current source having a sensing transistor and a bandgap circuit having first and second control transistors and a current mirror, the sensing transistor having a control electrode and a main current path, the main current path being connected to a feedback resistance at a first node, the other end of the feedback resistance being at a reference potential, each of the first and second control transistors having respective control electrodes, respective emitters and respective collectors, the first node being connected to the control electrodes of the first and second control transistors, the emitter of the first control transistor coupled to the reference potential via a first resistance and the emitter of the second control transistor coupled to the emitter of the first control transistor via a second resistance, the current mirror having a diode-connected transistor and a controlled transistor, the diode connected transistor connecting the collector of the first control transistor to a power rail and the controlled transistor connecting the collector of the second control to the power rail, the bandgap circuit being dimensioned to provide a first potential across said feedback resistance, characterised by a current amplifier having an input and an output, the input being connected to the collector of the second control transistor and the output being connected to the control electrode of the sensing transistor.
Preferably the first and second control transistors are of a first conductivity and the current mirror transistors are of a second opposite conductivity and wherein the current amplifier has a first amplifying transistor of said second conductivity having a control electrode connected to the collector of the second control transistor and a collector connected to the input of a second current mirror, said second current mirror comprising transistors having said first conductivity coupled to said reference potential.
Advantageously said second current mirror has an output connected to a diode-connected transistor of said second conductivity type, said output being further connected to the control electrode of said sensing transistor.
Conveniently the controlled transistor of the said current mirror has a first width and the amplifying transistor has a greater width.
Preferably the current source further comprises a start up circuit for the bandgap, the stare up circuit having a pull-up transistor for pulling said first node up to a second potential having a lesser magnitude than the first potential.
Advantageously said pull-up transistor is an emitter follower of said first conductivity and has a base connected to a voltage source comprising plural series diodes.
Conveniently said base is further connected to a switch for selectively shorting said diodes in response to a control signal.
Conveniently again said switch is an n FET.
Preferably said start up circuit further comprises a clamping transistor connected to the collector of the second transistor for selectively turning off said first amplifying transistor in response to said control signal.
Advantageously said clamping transistor is a p FET.
Preferably said switch is an n FET, and said start up circuit further comprises a p FET connected to the collector of the second transistor for selectively turning off said first amplifying transistor in response to said control signal, the current source having a control terminal for receiving a first voltage level operable to turn off said current source and a second voltage level operable to start said current source, said control terminal being connected to a control electrode of the p FET and to the gate of the n FET via an inverter.
Preferably again the current source has a plurality of second conductivity type output transistors, each having an emitter connected to said power supply rail, a base connected to the control electrode of the sensing transistor, wherein each of said output transistors has a collector providing a respective current output.
Advantageously at least one of said output transistors has greater width than another of said output transistors whereby said at least one output transistor provides a higher output current.